1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which feedback-controls the air-fuel ratio of a mixture supplied to the engine cylinder by cylinder, by means of feedback control to which an observer based on a modern control theory is applied.
2. Prior Art
Conventionally, there has been proposed an air-fuel ratio-estimating system for internal combustion engines, for example, by Japanese Laid-Open Patent Publication (Kokai) No. 6-173755, which uses an observer to estimate the air-fuel ratio of a mixture supplied to each cylinder, based on an output from an air-fuel ratio sensor arranged in a confluent portion of the exhaust system of the engine, for generating an output proportional to the air-fuel ratio of exhaust gases. According to the system, when the estimated air-fuel ratio falls out of a range defined by predetermined upper and lower limit values, the estimated air-fuel ratio is reset to an initial value (value corresponding to A/F=14.7), to thereby prevent the estimated air-fuel ratio from diverging.
The proposed air-fuel ratio-estimating system can thus prevent the estimated air-fuel ratio from diverging by resetting the same to its initial value when the estimate air-fuel ratio falls out of the range defined by the predetermined upper and lower limit values. The system, however, has the disadvantage that it takes a considerable time period for the estimated air-fuel ratio to converge again to a desired air-fuel ratio. Further, although the flow rate of exhaust gases influences estimation of the air-fuel ratio for each cylinder (hereinafter referred to as "the cylinder-by-cylinder air-fuel ratio"), a change in the rotational speed of the engine and the density of air (atmospheric pressure) which influence the exhaust gas flow rate are not taken into consideration.
Further, when an air-fuel ratio control amount is calculated based on the detected air-fuel ratio of exhaust gases, and the air-fuel ratio of a mixture supplied to the engine is feedback-controlled based on the thus calculated air-fuel ratio control amount, the air-fuel ratio control amount is normally set to a predetermined upper or lower limit value when the former falls outside a range defined by the upper and lower limit values. In such a case, however, the predetermined upper and lower limit values are fixed values, and therefore, if the above-mentioned method of setting the air-fuel ratio control amount is applied as it is to cylinder-by-cylinder air-fuel ratio feedback control based on the estimated cylinder-by-cylinder air-fuel ratio, it can result in degraded controllability of the air-fuel ratio, since the predetermined upper and lower limit values can assume unsuitable values depending on the change in engine rotational speed and air density (atmospheric pressure).